Port sort function for list of playlists to KF5.
Now that sorting is enabled, it turned out that the sort really was broken for everybody (sorry!). The old Qt 3 and 4 "sort by subclassing the list item" feature wasn't actually ported into the Qt 5 compatibilility list view. So although we were defining the compare function, nothing was actually using it. Things seemed to work fine on my local system but I suspect that's because I had playlists from KDE 4 times that I copied over which would have already sorted the special playlists to the beginning. Since the old method doesn't work I've just added a hidden sort key column for now. The "proper solution" is probably to use a QSortFilterProxyModel but that will be some time away. CHANGELOG:Restore proper sorting of the list of playlists. BUG:402398 FIXED-IN:18.12.1
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